In the manufacture of thermoplastic foams, a thermoplastic resin is rendered molten or fluid by the application of heat and/or mechanical working, and the molten plastic resin is then admixed under pressure with a volatile blowing agent, such as a fluorocarbon, to form a solution. This solution, of appropriate viscosity, is passed through a constricted orifice or die into a region of lower pressure, usually atmospheric pressure, causing the dissolved blowing agent to volatilize and form a mass of fine gas bubbles within the body of the fluid resin. Through a variety of processes, the chief of which is the loss of heat, the intimate gas/liquid mixture becomes a gas/solid foam.
Typically, the molten thermoplastic material is extruded through an annular die or orifice to form a foamed tube, which is then slit one or more times to form one or more flat foam sheets which are rolled up and stored for future processing. The stored rolls are subsequently unrolled and heated in an oven to a temperature sufficient to permit molding but too low to cause collapse of the foam. The heated sheet is molded into the desired shapes, and when sufficiently cooled to harden the structure, the completed articles are stamped or punched from the sheet. The unused remainder, or trim-scrap, is ground into small enough pieces to permit reprocessing as recovered resin. In typical manufacture, as much as 30-40% of the resin extruded as foam is recycled in this fashion.
Certain types of foam material, such as polystyrene foam for example, retain the volatile blowing agents within the cellular structure of the foam for considerable periods of time after extrusion. This presents an opportunity to recover the volatile blowing agent from the trim-scrap.
The recovery and reuse of the volatile blowing agent used in foam production would be of obvious economic benefit to the manufacturer, since the blowing agent is a major cost factor in the production of the foam. For example, the production of certain foam may use as much as 30 to 40 lbs. of blowing agent for each 100 lbs. of resin used. However, until the present invention, no process or apparatus has been developed which will permit recovery and reuse of the blowing agents contained in foam scrap materials. Thus, the conventional practice in commercial foam production processes has simply been to permit the blowing agents to escape into the atmosphere.
In recent years, there has been a great deal of concern over the harmful or potentially harmful effects of some volatile organic substances in the earth's atmosphere. In particular, there have been a number of studies directed to the effects of fluorocarbon emissions into the atmosphere, and efforts have been made to restrict fluorocarbon emissions in various industrial processes and in consumer items such as aerosol sprays. The present invention now makes it technically possible and economically feasible to collect and recover a significant portion of the fluorocarbon blowing agents in foam materials which would otherwise escape to the atmosphere.
In order to recover the blowing agent in a form suitable for reuse, the agent must be converted into the form that it will be used in the foam production process, i.e. a liquid, and the blowing agent must be free from contaminants, most notably air. To accomplish this separation from air, the main contaminant, is an important objective of the present invention.
Processes for recovering volatile compounds such as those used as blowing agents in foam production have been previously used in a number of industrial processes other than foam production. Generally, however, recovery of these materials has only been feasible in processes where it is possible to collect and recover the volatile material at a relatively high concentration without the presence of a large amount of other contaminants, such as air. Examples of several known recovery systems are disclosed in the below-listed U.S. patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,780,744 Neel et al December 25, 1973 3,788,331 Neel et al January 29, 1974 3,793,801 Tsao February 26, 1974 4,095,605 Conrad June 20, 1978 4,175,932 Durr et al November 27, 1979 4,289,505 Hardison et al September 15, 1981 ______________________________________